sort the dropdown filters

[polyester]

currently the dropdowns (select2 pattern) come in random order. That's confusing if you have a larger list.

I solved it locally by changing https://github.com/collective/collective.isotope/blob/master/src/collective/isotope/browser/templates/isotope.pt#L72 from

filter_values python:filter_data['values']">
to
filter_values python:sorted(filter_data['values']);">
but am not 'deep enough' into python to know if that could introduce edge cases or other problems. Works for me, though...

I can make PR if wanted.